1. Field of the Invention
The present invention relates to an image forming apparatus which reads a document as digital image data, compresses them to be stored in a memory device and prints the image after expanding the compressed image data in the memory.
2. Description of Prior Art
An image forming apparatus such as a digital copying machine reads a document as digital image, and the image data are stored in a memory device temporarily. After one page of the document is read, the image data are read from the memory device and supplied to a printer. It is also possible to edit document images, for example, to add page number, date or the like.
In a digital copying machine, the image data are compressed in the unit of page and stored in a code memory, in order to decrease a memory capacity. When printing is started, the data in the code memory are expanded in the unit of page and stored in an image memory. When one page of the document is expanded, the image data are read from the image memory and printed on a paper. Therefore, image output with the printer is started after all the image on one page is read and all the image data are expanded.
In a copying machine, in order to print a document fast, paper is supplied beforehand for printing when all the document is read. By supplying papers beforehand successively,- decrease in throughput due to delay of paper supply can be prevented. For example, in a first copy in a copying machine, the printer supplies a paper beforehand to a timing roller when start key is pressed, and a document is fed by a document feeder at the same time. When the document reaches to the read position, image read is started. By supplying the paper to the timing roller beforehand, image forming speed is improved.
Previously, image output to the printer is not started until all the image on one page is read and all the image data are expanded. Therefore, a time to be waited at the timing roller becomes long, especially for a document of large size such as A3, and the first copy speed is low.
Expansion time becomes longer and the time to be waited at the timing roller also becomes longer, for example, when compression ratio of document image is low or when a save copy mode is set where a plurality of document images are formed on a sheet of paper. Then, it is desirable that a next image output can be started always within a paper supply time in the printer.
If additional information such as page number is added to the data expanded in a page memory, image can be output only after the additional information is added to the expanded data. Therefore, when additional information is added to the image data, a time from the start of expansion to the start of output becomes longer, and copy speed becomes lower.